1. Silica Fume in Concrete

2. Cooperative Agreement
This slide presentation was produced under Cooperative Agreement DTFH61-99-X between the Federal Highway Administration and the Silica Fume Association.
This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof.
The contents of this report reflect the views of the author who is responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the Federal Highway Administration. This document does not constitute a standard, specification, or regulation.
Trade or manufacturers’ names that appear herein are cited only because they are considered essential to the objectives of the document. The Federal Highway Administration does not endorse products or manufacturers.
The Silica Fume Association was formed in 1998 to serve as a voice for producers of silica fume. Please visit the SFA web site ( for information on additional products produced under this cooperative agreement.
Cooperative Agreement

3. Main Outline Understanding Silica Fume Using Silica Fume in Concrete
Obtaining Silica-Fume Concrete
Working With Silica-Fume Concrete
Silica-Fume Concrete Projects
Main Outline.

4. Chapter 1. Understanding Silica Fume
Silica Fume Definitions
Silica Fume Production
Silica Fume Products
Silica Fume Reactions
Chapter 1 Outline.
Main
Outline

5. Chapter 2. Using Silica Fume in Concrete
Enhancing Mechanical Properties
Improving Durability
Enhancing Constructability
Producing High-Performance Concrete Bridges
Chapter 2 Outline.
Main
Outline

6. Chapter 3. Obtaining Silica-Fume Concrete
Specifying Silica Fume and SFC
Proportioning SFC
Producing SFC
Chapter 3 Outline.
Main
Outline

7. Chapter 4. Working with Silica-Fume Concrete
Conducting a Test Placement
Transporting, Placing and Consolidating
Finishing Flatwork
Finishing Bridge Decks
Preventing Plastic-Shrinkage Cracking
Curing
Chapter 4 Outline.
Main
Outline

8. Chapter 5. Silica-Fume Concrete Projects
Bridge Decks
Parking Structure
High-Rise Columns
HPC Bridge
Shotcrete Rehabilitation
HPC Constructability
Chapter 5 Outline.
Main
Outline

9. www.silicafume.org info@silicafume.org
Visit us at the addresses shown for additional information or to get the answer to your questions.

10. Chapter 1. Understanding Silica Fume
Silica Fume Definitions
Silica Fume Production
Silica Fume Products
Silica Fume Reactions
This chapter of the presentation explains what silica fume is and how it functions in concrete. Definitions of what silica fume is and is not, the production of silica fume at a smelter, the types of silica fume products, the physical and chemical properties of silica fume, and the reactions of silica fume in concrete are included.

11. Silica Fume Definitions
This section of the presentation defines silica fume. Additionally, several materials that are frequently mistaken for silica fume are presented.
Chapter
Outline

12. Silica Fume ... Very fine noncrystalline silica produced in electric arc furnaces as a byproduct of the production of elemental silicon or alloys containing silicon; also known as condensed silica fume or microsilica ACI 116R
ACI 116R-90, Cement and Concrete Terminology.

13. Silica Fume Summary
Smoke by-product from furnaces used in the production of ferrosilicon and silicon metals
Amorphous silica with high SiO2 content, extremely small particle size, and large surface area
Highly reactive pozzolan used to improve mortar and concrete
This slide sums up the properties and use of silica fume.

14. “Micropoz” (trademark)
Silica Fume (AKA)
Condensed silica fume
Microsilica
“Micropoz” (trademark)
Silica dust
Volatilized silica
These terms are frequently used for silica fume. Note that these are all describing the same material.

15. Silica Fume is NOT: Precipitated silica Fumed silica Gel silica
Colloidal silica
Silica flour
Precipitated silica, fumed silica, and gel silica are purposely made forms of silica. While these materials are amorphous and would be expected to perform well in concrete, they are typically too expensive for such use. More information on these materials may be found in ASTM E 1156 or in the work of Dunnom.
Colloidal silica is a stable dispersion of discrete synthetic amorphous particles of silicon dioxide. Colloidal silica may also be referred to as silica gel. Again because of the expense, colloidal silica is not being used in concrete.
Silica flour is a crystalline form of silica that may perform as a filler material in concrete. It will not contribute as a pozzolan.
ASTM E 1156, Standard Practice for Health Requirements for Occupational Exposure to Synthetic Amorphous Silica.
Dunnom, D., ed., 1984, Definitions for Asbestos and other Health-Related Silicates, STP 834, ASTM, West Conshohocken, PA.

16. Silica Fume Health Issues
The committee is not aware of any reported health-related problems associated with the use of silica fume in concrete.
--ACI 234R
The question of any health-related concerns resulting from using silica fume is frequently raised. The ACI committee report on silica fume states that there are no know health problems that have been attributed to the use of silica fume in concrete. However, there are certain precautions that should be followed. Specific recommendations are on the next slide.
ACI 234R-96. Guide for the Use of Silica Fume in Concrete.

17. Silica Fume Health Issues
Silica fume may contain trace amounts of crystalline quartz -- requires warnings on bags
Treat as respirable dust
Refer to materials safety data sheets (MSDS) for safety measures
Some amount of crystalline quartz is present in silica fume. Typically, this amount is below measurable limits. The presence of any amount of quartz requires warnings on bags and material safety data sheets for silica fume.
Silica fume should be treated as any other respirable dust; breathing large amounts of the dust should be avoided. Work areas should be well ventilated and appropriate dust masks should be worn. Manufacturers materials safety data sheets will provide specific recommendations.
Once the silica fume is incorporated into concrete, there is no potential for dust generation and no particular safety precautions beyond those typically required for working with fresh concrete are required.

18. Silica Fume Production
This section of the presentation describes the production of silica fume at a smelter. The raw materials going into the furnace, the electric-arc furnace, and the smoke collection system are shown. Finally, the uses of the metal produced in the process are given.
Chapter
Outline

19. Silica fume is a byproduct of producing silicon metal or ferrosilicon alloys in an electric-arc furnace. This photo, taken before environmental regulations were put into effect, shows silica fume being discharged from a smelter. Today, no silica fume is discharged to the environment in the United States.
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20. Silica Fume Production
Overview of the production of silicon metal or ferrosilicon alloys and the collection of silica fume. More details of the production process are given in the next few slides.
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21. Coal and quartz Wood chips
Raw materials going into the smelter are metallurgical grade quartz, coal, and wood chips. These materials are blended into a charge for the furnace. Handling and batching of these materials is not too dissimilar from procedures for batching concrete.
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Wood chips

22. Quartz gravel
Close-up view of the quartz aggregate used as a source of silicon.
No image available.

23. The charging deck of a furnace
The charging deck of a furnace. The tractor is being used to stoke the furnace charge. This is actually the cooler part of the furnace. At the bottom near the electrodes, the temperature is over 2,000 degrees C. The hood over the furnace is part of the collection system that collects the silica fume.
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24. A furnace being tapped -- molten silicon metal or ferrosilicon alloy is being discharged.
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25. The product that results from the smelting operation is simply metal that is sized and sold for further processing.
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26. Silicon metal and ferrosilicon alloys are further processed into a wide variety of industrial and consumer products. Silicone sealers and adhesives are just two of the many products that are developed from the product produced by the smelter.
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27. Schematic of the furnace and collection system
Schematic of the furnace and collection system. The charging deck shown previously is where the “A” is shown on the diagram.
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28. After being collected over the furnace, the silica fume must be transferred, cooled, and physically trapped. The large pipe on the left is bringing the silica fume from the furnaces. The vertical elements are cyclones that are used to remove oversize and other unwanted materials. The large building is the bag house where the fume is captured.
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29. Metals That Produce Silica Fume
Silicon metal - typically greater than 97% silicon
Ferrosilicon alloys -ranging from 40 to 90% silicon alloyed with iron
Silica fume from the production of silicon metal and ferrosilicon alloys may be suitable for use in concrete applications. However, silica fume from all of these products may not meet the minimum silicon dioxide requirements of ASTM C If a particular silica fume does not meet the requirements of ASTM C !240, testing is recommended to determine whether required concrete properties can be achieved.
ASTM C 1240, Standard Specification for Silica Fume for Use as a Mineral Admixture in Hydraulic-Cement Concrete, Mortar, and Grout.

30. Silica Fume Products Chapter Outline
This section of the presentation describes how silica fume is available for use in concrete. The various wet and dry product forms are described, including as-produced fume, slurried fume, and densified fume. Information on how these products are prepared is also presented. Blended cement containing silica fume is described. Pelletized fume, which is not suitable for use in concrete, is also described.
Chapter
Outline

31. Silica Fume Product Forms
As-produced powder
Water-based slurry
Densified
Blended silica-fume cement
Pelletized
Silica fume is available in all of the product forms shown in this slide. Further, these product types may be available in bulk, bags, or drums as appropriate for the material and for the size of the project. Except as noted below, there is no generally accepted data that indicates that any form of the product out-performs any other form when used in concrete. Each of these product forms is described in the next few slides.
Silica fume in the pelletized form is not suitable for direct use in concrete because the pellets will not break down and disperse in a concrete mixer. Pelletized silica fume can be interground with portland cement clinker to produce blended silica-fume cement.

32. Three product forms. The densified material is on the left and the as-produced material is on the right.
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33. Product Characteristics -- As-Produced Silica Fume
As produced directly from bag house
Extremely fine and dusty
Difficult to handle pneumatically -- sticky
Self agglomerating with a tendency to create small weak lumps
Low density yields small loads ( tons) (7 - 9 Mg) in bulk tankers
Because of the handling difficulties and the poor economics of transporting as-produced silica fume, it is rarely used in concrete. As-produced silica fume is frequently used in bagged construction products such as grouts and repair mortars.

34. As-produced silica fume
As-produced silica fume. This is silica fume as it comes from the bag house. While it can be used in concrete, this form is more difficult to handle and more expensive to transport because of its very low bulk density.
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35. Product Characteristics -- Silica-Fume Slurry
% solids (as-produced silica fume dispersed in water)
Storage tanks require agitation and protection from freezing
Transported in bulk tankers 4,000 gallons (12 tons of silica fume) (15 kL, 10 Mg)
General characteristics of slurried silica fume.

36. Slurried silica fume. Typically, the slurry consists of approximately 50 percent silica fume and 50 percent water, by mass. When first introduced to the market, slurried silica-fume products often contained chemical admixtures such as water reducers or high-range water reducers. Today, slurry is available without any such additions.
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37. Slurry storage tank at a manufacturer’s plant
Slurry storage tank at a manufacturer’s plant. This tank will hold 350,000 gal (1.3 ML). Use of this large tank accommodates the continuous operation of the furnaces and allows for blending of the slurry as it is produced.
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38. Product Characteristics -- Densified Silica-Fume
Reversible agglomeration process
Flows well pneumatically
Bulk transportation is economical, 22 tons (20 Mg) on a bulk tanker
Product density can be controlled for handling conditions and applications
General characteristics of densified silica fume.

39. Densified silica fume. The densified version of the material is easier to handle and more economical to transport than the as-produced fume.
Densified silica fume should not be confused with pelletized silica fume. In that process, water is added to the fume to form small pellets. These pellets become extremely hard and will not break down in a concrete mixer during concrete production.
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40. Densification silo for production of densified form of silica fume
Densification silo for production of densified form of silica fume. The as-produced fume is brought into this silo from the bag house. Compressed air is used to aerate and tumble the fume particles. Electrostatic charges develop and cause the individual particles to agglomerate. Once the densified silica fume reaches the desired bulk density, it is taken from this silo for packaging or shipping.
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41. Densified silica fume is frequently used in bulk at a concrete plant
Densified silica fume is frequently used in bulk at a concrete plant. Here, a bulk tanker is picking up a load of densified silica fume.
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42. A bulk tanker delivering a load of densified silica fume to a concrete batch plant. Bulk delivery of densified silica fume is very similar to bulk handling of portland cement or any other cementitious material. Compressed air is used to transfer the silica fume from the tanker into the silo at the plant.
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43. Product Characteristics -- Blended Silica-Fume Cement
Primarily used in Northeastern Canada, limited availability in US
Fixed silica fume content of 7.5% to 8.5%
Produced from as-produced, densified, or pelletized silica fume
Portland cement-silica fume blend is primary product. One blend of silica fume, fly ash, and portland cement now being marketed
To date, blended silica-fume cement has not been used to any real extent in the US. This lack of use may be attributed to these factors:
1) Lack of flexibility in silica fume dosage;
2) Limited volume of concrete that can be produced from one truckload of blended cement; and,
3) Difficulty in batching concrete for small projects.
Densified or pelletized silica fume must be interground during production of blended silica-fume cement to ensure dispersion of the agglomerates.

44. Product Characteristics -- Pelletized Silica Fume
Dustless
Non-reversible agglomeration
Small pellets, typically 3/8 to 1 inch (10 to 25 mm) diameter
Utilized in interground silica fume blended cement
Not suitable for direct use in concrete!
Silica fume producers have frequently pelletized silica fume as a means of preparing the material for disposal. This process involves adding moisture to the silica fume and results in very hard pellets. These pellets will not disperse during production of concrete in a central mixer or truck mixer.

45. Silica Fume Colors Premium -- White Standard -- Grey
Normal addition rates of silica fume may result in concrete that is slightly darker in color. For this reason, standard silica fume has not been used in architectural concrete when color is important. White silica fume may be used in this application.
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46. Silica Fume Reactions Chapter Outline
This section of the presentation describes how silica fume reacts in concrete. First, the pertinent physical and chemical characteristics of silica fume are given. Then, the physical and chemical effects of silica fume are addressed. This section concludes by looking at the effect of silica fume on the transition zone.
Chapter
Outline

47. Silica Fume -- Chemical Properties
Amorphous
Silicon dioxide > 85%
Trace elements depending upon type of fume
The amorphous structure and the very high silicon dioxide content are two of the keys to the performance of silica fume a pozzolan. There is no particular threshold for silicon dioxide content. Most of the silica fume used to date has been above 85 percent, and this value has been maintained in most specifications.

48. Silica Fume -- Physical Properties
Particle size (typical) < 4 x in.
Bulk density
(as-produced) to 27 lb/ft3
(slurry) to 12 lb/gal
(densified) to 45 lb/ft3
Specific gravity
Surface area (BET) ,000 to ,000 ft2/lb
Physical properties are important in both the micro-filler and pozzolanic roles of silica fume. The very small size of the silica fume particles is also one of the keys to its performance. The values for bulk density are of interest primarily for transportation. The very high surface area can be estimated using the nitrogen adsorption (BET) method only. Surface area determinations based upon sieve analysis are meaningless.
For terms of reference, 60,000 to 150,000 ft2/lb is about 1.4 to 3.4 acres of surface area per pound of silica fume.

49. Silica Fume -- Physical Properties
Particle size (typical) < 1 µ m
Bulk density
(as-produced) to 430 kg/m3
(slurry) to 1440 kg/m3
(densified) to 720 kg/m3
Specific gravity
Surface area (BET) ,000 to
30,000 m2/kg
SI version of previous slide.
Physical properties are important in both the micro-filler and pozzolanic roles of silica fume. The very small size of the silica fume particles is also one of the keys to its performance. The values for bulk density are of interest primarily for transportation. The very high surface area can be estimated using the nitrogen adsorption (BET) method only. Surface area determinations based upon sieve analysis are meaningless. SI

50. Typical particle size distribution
This plot shows a typical particle size distribution for an “as-produced” silica fume. The mean particle size for this sample was µm.

51. Cement grains (left) and silica fume particles (right) at the same magnification. The ACI 234R-96 report estimates that for a 15 percent silica fume replacement of cement, there are approximately 2,000,000 particles of silica fume for each grain of portland cement.
The longer white bar in the silica fume side is 1 µm long.
ACI 234R-96, Guide to the Use of Silica Fume in Concrete.
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52. Comparison of Chemical and Physical Characteristics -- Silica Fume, Fly Ash and Cement
Silica Fume Fly Ash Cement
SiO2 Content
Surface Area m2/kg 17, ,
Pozzolanic Activity
(with cement, %) n/a
(with lime, psi) (MPa) 1, , , n/a
( ) ( )
This slide presents a comparison of some of the physical and chemical characteristics of silica fume, fly ash, and portland cement.
Note that the surface area measurements for silica fume are done using a nitrogen absorption method (BET) while those for fly ash and portland cement are typically done using an air permeability method (Blaine).
Note also that the pozzolanic activity testing was done using a constant water content for the silica fume mixtures rather than a constant flow.

53. How Does Silica Fume Work in Concrete?
Physical effect
Chemical effect
Silica fume functions in concrete by two distinct mechanisms. As noted, the keys to its performance in these roles are its very small particle size and its high silicon dioxide content. Each of these effects is discussed in the following slides.

54. Silica Fume: Physical Effect
The presence of any type of very small particles will improve concrete properties. This effect is termed either “particle packing” or “micro filling”.
Several researchers have looked into the improvement in concrete properties resulting from including particles smaller than portland cement grains. The first person to promote the use of silica fume in this role was Hans Bache of Aalborg Portland in Denmark. Two of his papers are referenced in subsequent slides.

55. Physical Effect
The carbon black and plain cement mixes showed comparable strengths at both 7 and 28 days, even though the carbon black mixes contained 10 percent less cement (by mass) physical mechanisms do play a significant role, particularly at early ages.
-- Detwiler and Mehta
ACI Materials Journal
Detwiler and Mehta used carbon black particles to examine the filler effect. These particles are not pozzolanic and are approximately the same size as silica fume.
Rachel J. Detwiler and P. Kumar Mehta, “Chemical and Physical Effects of Silica Fume on the Mechanical Behavior of Concrete,” ACI Materials Journal, Vol. 86, No. 6, pp , 1989.

56. Schematic showing the dispersion of silica fume particles among cement grains. This figure shows the basic concept of particle packing -- filling the spaces between cement grains with silica fume particles.
H. H. Bache, “Cement-bound materials with extremely high strength and durability,” brochure form Aalborg Portland, Aalborg, Denmark, undated.
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57. Silica Fume: Chemical Effect
Silica fume is simply a very effective pozzolanic material

58. What is a Pozzolan?
A siliceous or siliceous and aluminous material, which in itself possess little or no cementitious value but will, in finely divided form and in the presence of moisture , chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.
-- ACI 116R
ACI 116R-90, Cement and Concrete Terminology.

59. portland cement + water = calcium silicate hydrate + calcium hydroxide
First step in the pozzlanic reaction.

60. pozzolan + calcium hydroxide + water = calcium silicate hydrate
Second step in the pozzolanic reaction. The CSH gel formed in this reaction is essentially identical to that formed in the initial reaction of portland cement and water.

61. This figure shows the reduction in calcium hydroxide content of a mature cement-silica fume paste with a water-cement ratio of This information is further evidence of the pozzolanic nature of silica fume.
From: Sellevold, E. J., Bager, D. H., and Klitgaard Jensen, E., “Silica-Fume Cement Pates: Hydration and Pore Structure,” Condensed Silica Fume in Concrete, Report BML , Norwegian Institute of Technology, Trondheim, 1983, pp
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62. The Transition Zone The transition zone is a thin layer between the bulk hydrated cement paste and the aggregate particles in concrete. This zone is the weakest component in concrete, and it is also the most permeable area. Silica fume plays a significant role in the transition zone through both its physical and chemical effects.
P. K. Mehta describes the transition zone in his textbook, Concrete: Structure, Properties, and Materials.
ACI Committee 234 describes the role of silica fume in the transition zone in the committee document, ACI 234R-96, Guide to the Use of Silica Fume in Concrete.

63. Schematic of the transition zone
Schematic of the transition zone. Note that in the area adjacent to the surface of an aggregate particle, the relatively uniformly sized cement grains are not packed as densely as they are in the bulk zone. This “wall effect” is one of the reasons stated for the relative weakness and permeability of cement paste in the transition zone.
When silica fume is added to the concrete, better particle packing occurs in this area. The silica fume in this area can further refine the transition zone through its pozzolanic reaction.
This sketch is taken from H. H. Bache, “Densified Cement/Ultra-Fine Particle-Based Materials,” Presented at the Second International Conference on Superplasticizers in Concrete, 1981.
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64. Socket where a sand grain has been pulled away from cement paste in 1-day old mortar. The sand grain was originally at the top of the picture. Note the open structure and the presence of crystals of calcium hydroxide in this region.
“In portland cement mortars, the microstructure of the interfacial zone, extending to about 20 to 50 µm from the sand grain surface, is significantly different from that of the bulk paste matrix away from the sand grain. It is characterized by a massive CH layer engulfing the sand grain and by some channel type gaps.”
“The formation of this zone may be the result of the presence of some water-filled gaps around the sand grains in the fresh mortar. These gaps may be the result of bleeding and inefficient filling with cement particles of the 20-µm space around the grain surface.”
Arnon Bentur and M. D. Cohen, “Effect of Condensed Silica Fume on the Microstructure of the Interfacial Zone in Portland Cement Mortars,” Journal of the American Ceramic Society, Vol. 70, No. 10, pp , 1987.
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65. Socket where a sand grain has been pulled away from cement paste in 28-day old mortar containing silica fume. The sand grain was originally at the top of the picture. Note the dense structure without the gaps and crystals seen in the previous photo.
“When 15% CSF by weight of cement is added to the mortar, the microstructure of the interfacial zone is significantly changed. Its structure is homogeneous and dense without the presence of a massive CH rim or gaps. These changes can be the result of the suppression of bleeding in the fresh mortar and the ability of the CSF particles to fill the space in the vicinity of the sand grain surface much more efficiently than the bigger cement particles.”
“The effect of CSF on densifying and homogenizing the microstructure of the interfacial zone may have a considerable influence on the performance of mortars and concretes. Therefore, when the effect of CSF on mortars and concretes is being considered, the interfacial effects should be taken into account in addition to the influence of the CSF on the bulk paste matrix.”
Arnon Bentur and M. D. Cohen, “Effect of Condensed Silica Fume on the Microstructure of the Interfacial Zone in Portland Cement Mortars,” Journal of the American Ceramic Society, Vol. 70, No. 10, pp , 1987.
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66. The improvement in the structure of the transition zone can be seen when broken test specimens are examined. Usually for high-strength, silica-fume concrete, most of the aggregate particles will be fractured, very few will be pulled out of the matrix. The better bonding of the aggregate into the matrix results in higher concrete strength and modulus.
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67. This slide summarizes the effects of adding silica fume to concrete.

68. End of Chapter 1
Main
Outline